1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Description of the Related Art
Polyimide is an excellent material in mechanical strength, thermal stability, and electrical insulating properties. Therefore, utilization is expected as an insulating material, a sealing material, a protective film, and the like in a semiconductor field and other various fields. As for a method of forming a polyimide film, known is a method, wherein imidization is induced by applying a solution of precursor polyamic acid to a substrate and performing drying and a heat treatment.
Herein, it is to be noted that concave portions with large aspect ratios and the like are often present on the surface of the substrate including a semiconductor element. It is difficult to allow the solution of polyamic acid to enter or guide the concave portions with large aspect ratios uniformly by the method in which the solution of polyamic acid is applied. As a result, voids are very often generated in the polyimide film.
Another method is also known, wherein a pressure is applied and, thereby, the solution of polyamic acid is allowed to enter the concave portions. However, a pressure is locally applied to a semiconductor substrate, so that an element may be damaged. Even if the solution of polyamic acid is allowed to fill, it is difficult to ensure a required film thickness because of reduction in volume due to vaporization of an organic solvent. In addition, bubbles may be generated in vaporization of the organic solvent and a load on the environment may be heavy because a large amount of organic solvent is vaporized.
Meanwhile, in recent years, attention has been directed to a technology, wherein a plurality of semiconductor substrates are three-dimensionally stacked. However, it is necessary that an insulating material is filled in between the stacked semiconductor substrates. At present, a side-fill method is used, wherein an epoxy based resin is poured in from a chip end portion taking advantage of capillarity. However, the suitability for mass production is poor because of a long treatment time, and there are problems from the viewpoint of the heat resistance and the mechanical strength. Polyimide has suitability as a filling material. However, in the case where, for example, silicon wafers having a diameter of 300 mm are stacked at an interval or a gap of 5 to 10 μm, it is substantially impossible for the conventional method by using an organic solvent to fill a polyimide film into such a wide region with a small interval or gap.
A vapor deposition polymerization method has been studied as a method for forming a polyimide film without using an organic solvent (refer to Japanese Unexamined Patent Application Publication No. 2000-3910, for example). In the vapor deposition polymerization method, a raw material monomer is vaporized and is supplied to a substrate surface and, then, polymerization is induced.
In addition, it has been studied to form an acrylic polymer by using carbon dioxide in a supercritical state as a solvent (refer to Japanese Unexamined Patent Application Publication No. 01-163203, for example). Meanwhile, the present inventors found that diamine and tetracarboxylic acid dianhydride were able to be dissolved into carbon dioxide in a supercritical state (refer to Naoya Fukui, Fumiya Kobayashi, Masashi Haruki, Shin-ichi Kihara, Shigeki Takishima “12th SCEJ Students Meeting Preprint” p 9, 2010, for example).
However, the vapor deposition polymerization method is not suitable for mass production because diamine and tetracarboxylic acid dianhydride serving as raw materials for the polyimide have low sublimation pressures (vapor pressures) even at high temperatures, so that the amount of supply of raw material monomers is reduced to a low level and the film formation rate is very low. In addition, polymerization of the raw material monomers occurs even at room temperature, and when two raw material monomers are mixed, a polyamic acid is generated. The polyamic acid has a very low sublimation pressure and is precipitated immediately. Therefore, it is substantially difficult to supply the generated polyamic acid in a vapor phase to the substrate surface. As described above, it is impossible to fill fine concave portions and gaps by the method in which precipitated polyamic acid is accumulated.
Meanwhile, Japanese Unexamined Patent Application Publication No. 01-163203 discloses that an acrylic polymer is formed by using carbon dioxide in a supercritical state as a solvent, although the material is different from polyimide, and the properties are different.